This invention pertains generally to lens arrangements for radiant energy and particularly to reflective, or catoptric, lens arrangements.
It is known in the art that either refractive or reflective elements may be arranged to focus or collimate "wave-propagated" energy, referred to herein as radiant energy. That is, wave-propagated energy such as electromagnetic energy or acoustic energy may be focused, or collimated, by lenses made from refractive materials or from reflective materials.
When refractive elements are used, focusing or collimating is accomplished by reason of the deflection from a straight path of the radiant energy in passing obliquely from one refractive element to another. Therefore, by judiciously selecting the material from which the refractive elements are fabricated and properly shaping such elements, lens systems may be made which satisfactorily focus or collimate radiant energy in many applications. There are, however, inherent limitations of refractive lens systems which militate against their use in many cases. For example, because the material from which refractive elements are made absorbs an appreciable portion of the radiant energy, unacceptable losses are experienced when the power level of the radiant energy is either very low or very high. When the amount of radiant energy is very low, any absorption of radiant energy in the lens elements is, of course, undesirable. When the amount of radiant energy is very high (as is the case, for example, when a laser beam is involved absorption of the radiant energy in the lens elements may be great enough to cause excessive heating of the lens elements so that such elements are rendered useless or even destroyed.
The foregoing difficulties may be almost completely avoided by using reflecting elements in a catoptric lens system. Unfortunately, however, other problems are encountered when reflecting elements are so used. First of all, the problem of aperture blockage is encountered. That is, in any known catoptric lens system, as the conventional Cassegrainian lens system wherein the focal points of cooperating reflecting elements are disposed along a lens axis, one of the reflecting elements obscures radiant energy to some degree, thereby causing vignetting. Another problem with any catoptric lens system is the fact that the reflecting elements must be spaced from each other. As a result, unless folded paths are provided for radiant energy within a catoptric lens system, the physical size of such a system must be, relatively speaking, much larger than the physical size of a refractive lens system having a comparable aperture.
"It has been proposed to provide, in a catoptric lens arrangement for radiant energy, a primary mirror in the form of a convex paraboloid and a secondary mirror in the form of a concave ellipsoid. When such mirrors are mounted with respect to each other so that: (a) the focal point of the primary mirror is coincident with one of the foci of the secondary mirror; and, (b) the xis of symmetry of the primary mirror is not coincident with the major axis of the secondary mirror, radiant energy in an incident paraxial beam is focused, to some degree, at the second focal point of the secondary mirror. It has been found, however, that a catoptric lens arrangement of this type is subject to the achromatic aberrations, i.e. spherical aberration, coma, astigmatism (or curvature of the field) and distortion. Such aberrations prevent the arrangement from being "diffraction limited".
As is known, it is not possible to eliminate all of the different achromatic aberrations from a simple catoptric lens. However, by constructing a compound lens, it is possible to balance the aberrations of different ones of the lens elements so as to reduce the overall effects of the various kinds of aberrations. Even when great care is taken, however, residual aberrations remain significant enough that, in designing high quality lenses, it is usually necessary to decide which aberrations are most detrimental to the purpose for which a particular lens arrangement is intended and then to reduce only the most detrimental aberrations to a negligible amount. Thus, in a telescope objective which is required to cover only a small angular field, spherical aberration and coma are the most important of the achromatic aberrations. Such aberrations may be reduced (while maintaining resolution) by using an objective lens with a long focal length and a small f-ratio. Following known design techniques to obtain a lens arrangement with the combination of a long focal length and a small f-ratio requires a physically large structure. Attendant mechanical and thermal problems must, therefore, be overcome to provide the required structural stability of the telescope. On the other hand, the objective lens for the ordinary camera, which covers a relatively large field, may be only partially corrected for spherical aberration and coma because of the greater importance of correcting astigmatism, curvature of field and distortion.
The difficulty in compromising between the various kinds of achromatic aberrations in the design of known less arrangements arises, in great measure, from the heretofore accepted theory that, in any compound lens arrangement, the real and virtual foci of each individual lens element must fall on the lens axis.